So, when I tested the new feature, the captions did not appear in the viewer and could not be exported. I scratched my head as to why this was. I watched some videos, and they seemed to just be able to drop them in and have them work.
I figured out why captions didn't work with my projects. My projects are at a non-standard resolution: 3840 x 1600. I work this way because it is much more clean and nice to have a proper 2.4:1 aspect ratio rather than add some faux-letterbox nonsense into a 3840 x 2160 timeline. But the captions standard CEA-608 does not support non-16:9 ratios it seems. Also, caption exports are not allowed with custom Compressor presets (!)
There is a work around. Time and sync your captions in the original timeline. Export this timeline with your custom resolution and Compressor preset. Create a new project in one of the standard preset resolutions. Copy paste the entire original timeline into the new project. Export the project with the standard "Export File..." dialog, and select the Roles pane. Select "Export each ITT language subrole as a separate file". You can export this video at whatever quality you want: I choose h.264, because we don't actually care about the video quality of this second project, we just want the captions file from this export and h.264 will be the quickest. When both exports are done, delete the h.264 standard aspect ratio video component. Upload the export that was done with your custom settings to YouTube, and then when that upload is done, just upload the .itt file from your second export to Youtube. And bam! You have all of your captions, perfectly synced, and with proper export settings.
This would be simplified if apple had allowed a simple "Export captions" dialog, but sadly we must do this obtuse and time wasting export of some proxy nonsense file that immediately gets sent to the trash upon birth, just so we can get a upload-able .itt file. I just enjoy how Apple added a section that would troubleshoot caption errors for you, but I didn't get any error telling me that custom resolutions do not work with captions, they were just greyed out. I had to figure this all out by myself. As for non-YouTube exports, well I guess you're screwed if you want to do any custom resolutions because captions are just not allowed within the CEA-608 standard. Dumb.
I figured out why captions didn't work with my projects. My projects are at a non-standard resolution: 3840 x 1600. I work this way because it is much more clean and nice to have a proper 2.4:1 aspect ratio rather than add some faux-letterbox nonsense into a 3840 x 2160 timeline. But the captions standard CEA-608 does not support non-16:9 ratios it seems. Also, caption exports are not allowed with custom Compressor presets (!)
There is a work around. Time and sync your captions in the original timeline. Export this timeline with your custom resolution and Compressor preset. Create a new project in one of the standard preset resolutions. Copy paste the entire original timeline into the new project. Export the project with the standard "Export File..." dialog, and select the Roles pane. Select "Export each ITT language subrole as a separate file". You can export this video at whatever quality you want: I choose h.264, because we don't actually care about the video quality of this second project, we just want the captions file from this export and h.264 will be the quickest. When both exports are done, delete the h.264 standard aspect ratio video component. Upload the export that was done with your custom settings to YouTube, and then when that upload is done, just upload the .itt file from your second export to Youtube. And bam! You have all of your captions, perfectly synced, and with proper export settings.
This would be simplified if apple had allowed a simple "Export captions" dialog, but sadly we must do this obtuse and time wasting export of some proxy nonsense file that immediately gets sent to the trash upon birth, just so we can get a upload-able .itt file. I just enjoy how Apple added a section that would troubleshoot caption errors for you, but I didn't get any error telling me that custom resolutions do not work with captions, they were just greyed out. I had to figure this all out by myself. As for non-YouTube exports, well I guess you're screwed if you want to do any custom resolutions because captions are just not allowed within the CEA-608 standard. Dumb.